In recent years, with the aim of strengthening information security, high value services, and automation, demand for replacing bar code systems that are applied to distribution systems and magnetic card systems for cashing, commuter passes, or the like, with automatic ID recognition systems that use an IC card or an IC tag, has been increasing. Among these systems, there are one in which the exchange of data and supply of power occurs without directly contacting the reader, that is, wirelessly, and such systems are referred to as “noncontact RF ID systems”.
Noncontact RF ID systems are divided into a close coupling type, which is coupled to the reader, a proximity type, which is used separated by about 20 cm, and a remote type, which is used separated by about 50 cm or greater.
The close coupling type is used generally for credit cards or the like, while the proximity type is used for commuter passes, ID cards, and the like. Remote type is used for tags in logistic systems or the like. The close coupling and proximity types receive information and a power supply by using a magnetic field. The remote type receives supplies of these by radio waves. Among these three types of noncontact RF ID systems, as receiving power of the remote type is very weak in especially, the remote type has a development theme of a low power consumption operation and a highly efficient power supply.
FIG. 11 shows a configuration of a conventional noncontact RF ID system. The noncontact RF ID system provides a reader 1 and a transponder 2. The transponder 2 provides an antenna 2A, a DC power detecting circuit 200, a signal detecting circuit 201, an input amplifier 202, a clock generating circuit and a demodulator 203 that use a phase locked loop and a reference circuit, a control logic circuit 204, and a memory 205.
The DC power detecting circuit 200 provides a diode D1, a diode D2 for a power supply, and a capacitor C1 for power storage. The signal detecting circuit 201 provides a diode D1, a waveform detecting diode D3, a load capacitor C2, and an FET switch Q1.
In the above structure, an amplitude modulation signal that includes clock and data information is sent by the reader 1 to the transponder 2 via the antenna 1A. In the transponder 2, when the signal is received via the antenna 2A, an electrical charge is accumulated in the power accumulation capacitor C1, the voltage at both terminals of the capacitor C1 serves as an electromotive force, and the transponder 2 is activated.
The signal detected by the waveform detecting diode D3 in the signal detecting circuit 201 is divided into the data component and the clock component by the clock generating circuit and the demodulator 203, and processed by the control logic circuit 204. When the transponder 2 responds to the reader 1, the response is carried out by turning the FET switch Q1 ON and OFF, and modulating the impedance of the antenna 2A by using the load capacitor C2.
In the conventional noncontact RF ID system, for example, as is described in the following non-patent document 1, Manchester encoding is applied to the exchange of data between a transponder and a reader.
[Non Patent Document 1]
D. Friedman, et al., A Low-Power CMOS Integrated Circuit for Field-Powered Radio Frequency Identification Tags, IEEE, ISSCC97 SA. 17.5, 1997
FIG. 12A shows a waveform modulated by the Manchester encoding. The Manchester encoding assigns a code “1” to a transition from a high level (high voltage state) to a low level (low voltage state), and assigns a code “0” to a transition from a low level (low voltage state) to a high level (high voltage state).
Here, in the case in which the time intervals between the high level and the low level are not equal, that is, in the case in which the duty ratio is not 50%, a DC offset is generated by the data, and this is a significant cause of reading errors when the received signal level fluctuates. The Manchester encoding sets the time intervals between the high level and the low level equal, realizes a signal having a 50% duty ratio, does not generate a DC offset, and thereby realizes a code suitable for communication.
However, in the demodulation of the Manchester encoding in the conventional example described above, because the codes “0” and “1” are determined by the sequence of the appearance of the high level and low level states, it is necessary to detect each of the high and low level states.
In addition, as shown in FIG. 12B, because the intervals of the state transition timing of the fall and rise fluctuates corresponding to the data, a phase locked loop and an oscillator become necessary to generate the clock signal, and therefore the convergence of the clock takes time. To satisfy the locking conditions of the phase locked loop, it is necessary to cancel the fluctuations of the temperature, power supply voltage, device processes, and the like. Therefore, a complicated reference circuit becomes necessary, and the consumed current increases. In cases where the communication is temporarily stopped due to the influence of the state of the radio waves, there are the problems that the convergence of the clock takes time and a long locking time is required.
Another conventional noncontact RF ID system has been proposed in which the code is generated without complicated phase locked loops or oscillators, a reference clock for an integrated circuit on the transponder is generated, and a code that satisfies a duty ratio of 50% is used (refer, for example, to Japanese Unexamined Patent, First Publication No. H11-355365).
FIG. 13A shows the waveforms and codes used in communication with another conventional noncontact RF ID system. As shown in FIG. 13B, the time intervals of the rising of the waveforms that are transmitted or received are equal.
The transmitted and received waveforms are obtained by a combination of waveform A and waveform B. The waveform A is one that extends the high level state in the positive time direction by T/2 (where T is 1 cycle) from the point in time that the waveform rises and extends the low level state in the negative time direction by T/2. The waveform B is one that maintains the high level state in the positive time direction for time t1 from the point in time that the waveform rises, maintains a low level state for time t2 until reaching the end point of the waveform, maintains the low level state in the negative time direction for t1 from the point in time that the waveform rises, and maintains the high level state for the time t2 until reaching the starting point of the waveform.
It is assumed that t1+t2=T/2, and both waveforms A and B necessarily have a rising state transition present at the center. If each of the independent waveforms A and B is respectively assigned “0” and “1”, then as shown in FIG. 14, when the waveforms B continue in succession, a rising state transition occurs at the junction between the waveforms, and thus associating the rising timing with one unit of data becomes difficult. In the case in which the waveforms B continue in succession, a rising state transition at the junction between the waveforms occurs because the waveform B starts at a high level and ends at a low level.
In another conventional examples, a code “0” is assigned when the waveforms A continue in succession twice, while a code “1” is assigned when the waveform A continues in succession after the waveform B. In this case, two successive waveforms A associated with code “0” start at a low level and end at a high level, and successive waveforms B and A associated with the code “1” start at a high level and end at a high level. The four combinations of all possible junctions, “00”, “01”, “10” and “11”, are shown in FIG. 15A to FIG. 15D.
In the case of the codes “00” and “10”, a falling transition occurs at the junction between the two successive waveforms. In the case of the codes “01” and “11”, the two junctions of the two successive waveforms are maintained at high level. Even if waveforms associated with sequence of arbitrary codes “0” and “1” are arranged, no rising transition occurs at the junction between the waveforms. Therefore, a rising transition always occurs only at the center points of each of the waveforms A and B. By using a circuit that detects the rising transition, a clock signal that is in synchronism with the data can be easily generated.
As shown in FIG. 16, the combinations of waveforms A and B can have many variations, such as the case in which when the waveforms A and B are switched and waveform B continues in succession after waveform A, then a code “1” is applied. A combination in which the time interval of the rise is constant, is realized by a combination of a waveform pattern that starts at a low level and end at a high level and a waveform pattern that starts at a low level or a high level and ends at a level identical to the starting level.
As has been described above, in another conventional example, by combining two types of waveforms having a duty ratio of 50%, the time intervals of the rising and falling times are made equal, and at the same time, it is possible to send the information for codes “1” and “0”. If the state transition that occurs at equal time intervals is used as a trigger, a clock that is in synchronism with the data can be easily obtained without using a phase locked loop.
However, when assigning the codes “0” and “1” to each of the waveforms A and B independently, as shown in FIG. 14, when the waveforms B continue in succession, a rising state transition occurs at the junction between the two waveforms B, and thus there are problems in that associating the rising timing with one unit of data is difficult, and the transmission efficiency due to the encoding declines.
In consideration of the problems described above, it is an object of the present invention to provide a communication method for a noncontact RF ID system, a noncontact RF ID system, and a transmitter and receiver that improve the transmission efficiency by encoding.